1. Field of Invention
The present invention relates to catalytic conversion of exhaust gases for internal combustion engines, and more particularly to improved diagnosis and efficiency performance characteristics of catalytic converters using various control systems and methods.
2. The Related Art
Various new techniques are being developed for reducing total vehicle exhaust emissions over an automobile's lifetime. An emerging field of invention pertains to the sensing and control of an engine's individual cylinders' for improving synergies with catalytic converter operation to minimize toxic air pollutants entering the atmosphere. Previously, most approaches focused on controlling the aggregate of exhaust gases or total output from all engine cylinders, such as the typical application of using one oxygen sensor to provide closed loop fuel control feedback for multiple cylinders. Using the aggregated engine emissions reduction approach requires greater system costs due to increases in catalyst size or precious metal loading or necessitates adding additional hardware, such as external systems for exhaust gas recirculation or secondary air injection, for realizing further reductions in automobile tailpipe pollutants. This aggregate technique becomes increasingly expensive, as regulated automobile air pollutant levels are approaching a zero emissions level, while mandated vehicle emission system warranty lifetime periods are being greatly increased. These two contradictory requirements of reducing engine emissions towards zero over longer warranted lifetimes have thus necessitated significantly improved on-board vehicle diagnostics methods, particularly for catalysts and catalytic converters. As the mandated warranted life of an automobile increased towards equivalent periods reaching between 10 and 20 years, the importance of onboard diagnostics has become critical for minimizing air pollution emitted over an automobile's useful lifetime. Both diagnostics and the rapid heating of an automobile's catalytic converter immediately following cold engine starts have become very important techniques for reducing the total pollutants generated by an internal combustion engine.
An emerging field of invention is focused on increased synergy between engine and catalytic converter operation. Increased synergy is provided by controlling all conditions of individual engine cylinder's output gases plus how these conditions can aid the catalyst in reducing total air pollutants. Adopting such a synergistic approach allows smaller catalytic converters, using reduced precious metals, mounted closer to the engine's cylinders for providing quicker catalyst heating after cold starts. Rapid catalyst heating is one important area since a major portion of total vehicle emission of toxic pollutants can be produced during the first few minutes when the engine is cold. Many of the recent reductions in air pollutants, from newer automobiles, are occurring by more quickly heating a catalytic converter and thereby converting pollutants into non-toxic gases sooner after engine starting. However, these improvements in engine control and catalyst synergy, such as early catalyst heating technology, also results in the overall reductions of toxic air pollutant now becoming more dependent on an automobile's catalytic converter. Early catalyst heating further increases the dependency on the catalytic converter since most air pollution is created during the first 60 seconds following cold engine starts.
High pollution levels, after engine cold starts, result because most catalyst surfaces must reach temperatures above 100° C. before becoming activated and allowing conversion of toxic exhaust gases into non-toxic forms. Therefore reducing time delays before a catalyst's surfaces reach temperatures allowing exothermic chemical activity further increases the dependency on proper catalytic converter operation for total emission reductions. Since more than 50 percent of total air pollutants can occur while the catalyst is too cold a typical approach has been to reduce the time the catalyst remains below chemical activation temperatures. However, this also increases dependency on the catalytic converter as the primary source of pollutant reductions when total amount of air pollutants eliminated by the catalyst increases. So now, in the event of catalytic converter deterioration, even more air pollutants are allowed into the atmosphere unless an accurate, cost effective catalyst diagnosis technique is available. These factors have increased the importance of more accurate, continuous diagnosis of the catalytic converter's pollution reduction performance to be allied with synergistic engine systems aiding improved overall catalyst performance. Improving the accuracy of onboard vehicle diagnostics provides added confidence that an automobile's tailpipe pollutants remain only slightly changed during the vehicles lifetime. It is especially advantageous to invent engine control techniques to further reduce overall toxic air pollutants exiting the catalyst while also being readily integrated with catalyst diagnostic systems, yet adding minimum engine hardware.
A number of techniques have been disclosed that use modifications of engine control software algorithms for providing early catalyst heating following a cold start. However, most of these techniques are possible only after the closed loop engine control system can be activated. As is known to one skilled in the art, an engine typically operates rich during the open loop fuel control period prior to closed loop engine control being enabled. As a result, shortly after engine cold starts, a zirconia, switching type oxygen sensor is usually inactive or saturated in the rich state beyond the sensor's range of stoichiometric detection and thus prevents feedback for individual cylinder fuel control by methods taught previously. Delaying catalyst heating, necessitated by the activation of the closed loop fuel system, allows additional pollutants to be exhausted from an automobile's tailpipe. Examples of some methods requiring the activation of the oxygen sensor and closed loop fuel control system for allowing catalyst heating include U.S. Pat. Nos. 5,974,785, 5,462,039, 5,661,971, 6,202,406 and 5,974,790.
Activating catalyst heating during the period of open loop fuel control operation, prior to enabling cold loop fuel control, is particularly advantageous in reducing tailpipe emissions. This is because more than half of total vehicle pollutants can be generated during the first minute of cold engine operation. However, all the earlier teachings of catalyst heating techniques solely dependent on an oxygen sensor to provide air-fuel ratio cycling are therefore unable to operate during open loop fuel control while the cold engine runs rich for stabilizing operation and preventing stalls. In addition, the magnitudes of catalyst heating possible are typically limited in these prior teachings since they are dependent upon closed loop fuel control operation where fuel cycling perturbations beyond 10% may lead to engine control stability concerns. These stability issues result because a typical switching type zirconia oxygen sensor can only detect gas characteristics within a few tenths of one air-fuel ratio around the stoichiometric control point. Outside this narrow stoichiometric range, a switching type oxygen sensor can only resolve rich or lean characteristics. Operation outside this narrow range leads to control feedback gain instability because the switching oxygen sensor can no longer differentiate actual air-fuel ratio errors using the methods taught previously. U.S. Pat. Nos. 5,675,967 and 6,202,406 require the oxygen sensor to provide output, indicating operation near stoichiometric, and this is typically unavailable in systems disclosed in previously using switching type oxygen sensors during rich engine operating conditions.
It is particularly beneficial to be able to heat the catalyst as early as possible following cold engine starting without adding costly engine hardware. Providing techniques having simple and reliable diagnostic methods are also advantageous for assuring proper system operation for minimizing air pollution. A number of techniques have been disclosed in various patents for improving catalyst heating. However, all the related rapid heating techniques disclosed in prior patents, such as U.S. Pat. Nos. 5,974,790, 5,974,785, 5,661,971, 5,600,948, 5,462,039, 5,357,928, fail to teach methods of early catalyst heating during the time period preceding engine closed loop fuel control activation. Further, heating methods dependent on closed loop fuel control become increasingly ineffective as lower thermal mass catalysts are moved closer to the engine exhaust manifold. These low thermal mass catalysts become chemically active within the first 30 seconds of engine cold starting while closed loop fuel control may still be disabled. All the aforementioned references are dependant upon waiting until closed loop fuel control is activated and are thus nonfunctional during the critical time period when an inactive oxygen sensor may allow only open loop fuel control operation. Each of these teachings also fail to disclose how control of each individual cylinders' exhaust gases is accomplished since they depend upon aggregate control of air-fuel ratios from a common oxygen sensor, and thus only during closed loop fuel control engine operation. There is no synchronization of the exhaust gases' measurements for each individual cylinder with corresponding control air-fuel ratio levels from these same cylinders, such as by correlation with the oxygen sensor's output, disclosed in these previous teachings to eliminate detrimental factors such as control system feedback time delays. Some of the previous techniques disclosed also require modifications or addition of new engine hardware to provide early catalyst heating, such as requiring an in-cylinder fuel injection system required in Kaneko's U.S. Pat. No. 6,041,591. While such added hardware provides additional benefits for the methods disclosed herein, such as extending lean limits of operation, they also require use of a more costly in-cylinder fuel injection systems.
Some methods to detect individual cylinder air-fuel ratio variations due to manufacturing tolerances and component degradation have been disclosed in teachings such as U.S. Pat. No. 6,148,808. These teachings require the use of a more costly wide range linear oxygen sensor to synchronize sampling of sensor output with the correct engine cylinders in order to adjust individual cylinder's exhaust gas errors closer to stoichiometric conditions for improving catalyst efficiency. U.S. Pat. No. 6,148,808 also proposes an alternate method of using a switching oxygen sensor by a method of successively indexing an estimated individual cylinder fuel correction and then detecting minimum A/F variations at the oxygen sensor. This method is disadvantaged by potentially long indexing periods before estimated individual cylinder fuel corrections are properly indexed with the correct cylinder and also sensitivity to unexpected A/F variations that can occur during normal engine operation. The focus of these teachings is to improve catalytic converter performance during steady state engine operation by reducing individual cylinder fuel injection quantity variations.
Some methods disclose modifications in engine fuel and spark ignition timing control to rapidly heat the catalytic converter shortly after a cold engine start. Typically, these approaches use significant ignition timing retard until closed loop fuel activation allows cycling between rich and lean exhaust conditions at the catalyst. Such methods provide limited improvements in catalyst exothermic heating because of waiting too long after cold starting and are primarily dependent upon enabling closed loop control thus limiting increases in rates of heating. In addition, dependence of these methods on closed loop control activation using switching type oxygen sensor feedback results in limited magnitudes of fuel changes due to both stability concerns and of causing increased pollutants if the catalyst hasn't reached sufficient temperatures. For example, increases in total air pollutants may result by activating the fuel control methods taught in U.S. Pat. No. 6,041,591 before the catalyst has reached temperatures sustaining exothermic chemical heating. More rapid heating using aggressive catalyst heating fuel control methods is allowed only during “active” catalyst conditions at temperatures above 400° C. to prevent the potential of increasing total air pollutants. Most catalyst heating is therefore provided by hotter engine exhaust gases caused by ignition retard, and results in increased fuel consumption.
Vehicle fuel economy can be improved by advancing ignition timing when using more effective, earlier enabled fuel control catalyst heating methods by actively detecting initiation of catalyst exothermic heating. Earlier enabling of fuel control changes for catalyst heating, as disclosed in this invention, provides significantly greater rates of heating than ignition retard without the undesired loss in fuel economy. Prior teachings disclosing methods using more aggressive fuel control changes to increase catalyst heating rates such a U.S. Pat. No. 6,041,591 can inject almost raw fuel into the catalyst. Such methods fail to detect initiation of catalyst exothermic heating and therefore can only be used when catalyst temperatures are above 400° C. to prevent the potential of higher air pollutants. Therefore these methods must wait too long after a cold start since they are disabled until reaching an estimated light-off catalyst temperature that can change significantly over operational lifetime. Otherwise, these catalyst heating methods may cause significantly increased air pollutants in the event that catalyst hasn't reached temperatures sustaining chemical exothermic heating. Previously disclosed methods of catalyst heating such as U.S. Pat. Nos. 5,249,560, 6,202,406 and 5,845,492 also wait too long before providing increased heating after a cold engine start. These methods use either too limited or more aggressive engine control techniques, such as disabling fuel to some cylinders, which can also cause significant engine roughness. U.S. Pat. No. 6,202,406 discloses a method of early heating until reaching an estimated catalyst light-off temperature by using a rich A/F, but with no method of providing oxygen to the catalyst for rapid heating, after a cold engine start. Use of an estimated catalyst light-off temperature, as taught in U.S. Pat. No. 6,202,406, can result in significant delays before methods of exothermic catalyst heating are enabled due to the widely varying temperature range for initiation of a catalyst's exothermic heating over operational lifetime. This method also waits too long for effective rapid early catalyst heating under some conditions by using only a static temperature threshold for enabling heating to approximate a widely varying range of light-off catalyst temperatures over a vehicle's lifetime. And U.S. Pat. No. 6,202,406 determining of catalyst light-off temperature based upon modeling data requiring A/F input dependency creates the potential of large diagnostic and fuel control errors when model data is inaccurate due to unanticipated operational conditions. These prior teachings fail to disclose methods of diagnosing catalytic converter performance and heating methods while the engine is operated under non-stoichiometric cold start conditions using a catalyst temperature sensor.
Other patents, for example U.S. Pat. Nos. 5,675,967, 5,715,676 and 6,202,406, disclose methods of diagnosing catalytic converter performance at approximately the time when a catalyst first becomes chemically active, shortly after a cold engine start. These disclosures teach methods of detecting a catalyst's light-off by comparing detection algorithm determination conditions to oxygen sensor fluctuation data or from temperatures obtained from either catalyst temperature modeling algorithms or fixed predetermined estimates of anticipated light-off temperatures. Use of oxygen sensor fluctuations or an estimated catalyst light-off temperature, as taught in all these US patents, can result in significant delays. This is because there is no oxygen sensor fluctuation data during non-stoichiometric engine operation when a switching oxygen sensor is saturated, for example, after cold starting when rich operation causes the sensor to be at its rich limit. Use of an estimated catalyst light-off temperature from modeling algorithms, before enabling methods of exothermic catalyst heating, also delays early heating due to the widely varying temperature range for initiation of a catalyst's exothermic heating over operational lifetime. And there are no accurate temperature modeling algorithms that are able to predict the changing temperatures when exothermic heating first occurs as a catalyst deteriorates over time. Therefore these prior methods result in higher total air pollutants since they cannot account for the wide range of actual, non-static factors influencing this catalyst activation temperature, such as aging and when condensation occurs on close coupled catalyst active surfaces during some normal environmental operational conditions and delay activation. Engine control methods based upon use of static catalyst temperature thresholds don't account for constantly changing operational conditions such as the delayed initiation of catalyst exothermic heating, such as catalytic converters deteriorate over their useful lifetimes. These prior teachings typically use modeled data or static enabling thresholds in the methods disclosed, such as use of coolant or a fixed catalyst temperature that can change significantly over a vehicle's operational lifetime and delay more rapid catalyst heating or diagnosis. It remains impractical to model catalyst temperatures during many actual operational conditions, such as delayed heating due to condensation on metal foil surfaces of the catalyst, that many prior methods depend upon for implementing catalyst diagnosis and heating. However, such conditions can have significant impact on actual total air pollution when non-adaptive methods control algorithms are unable to properly adapt for such factors as catalyst condensation during normal engine operation.
Catalysts for engine exhausts are used to convert unburned or partially reacted gases that are mostly made up of hydrocarbon (HC), carbon monoxide (CO), and nitrogen oxides (NOx) components. Gases leaving the exhaust manifold of an internal combustion engine enter the inlet of a device called a catalytic converter. A catalytic converter is the device made up of multiple catalyst elements that provides an expansive area where these gas components are oxidized to carbon dioxide (CO2), nitrogen (N2), and water vapor (H2O) by catalyst materials such as platinum (Pt), palladium (Pd), and rhodium (Rh). The conversion of these toxic gases into CO2, N2 and H2O results in an exothermic chemical reaction at the catalyst surface and causes an increase in the temperature of the gases leaving the outlet side of the catalyst element. Intentionally increasing the steady state concentrations of unburned or partially reacted gases entering the catalyst will result in a temperature rise that can be used to determine the catalyst's conversion efficiency as well as to heat it as described herein. Each catalyst may have different oxidation characteristics for more completely converting gases entering from the engine's exhaust manifold into less toxic gases.
Various methods for monitoring the catalyst's conversion efficiency by monitoring the level of the chemical or exothermic reactions occurring within a catalytic converter have been proposed. Detection of the catalyst's efficiency with a chemical method usually incorporates the use of two oxygen sensors placed at the inlet and the outlet portions of the catalytic converter catalyst elements being monitored. The voltage versus time characteristics of the two oxygen sensor's output signals provides an indication of the catalyst's ability for storing oxygen for chemical oxidation reactions. The catalyst's oxygen storage capacity causes a difference between the inlet and outlet oxygen sensor signals. Since the catalyst's oxygen storage capacity decreases after extended high temperature operation, the sensors output voltage versus time characteristics become more similar as the catalyst's ability to store oxygen drops to zero. The accuracy of oxygen sensor based diagnostic methods have become less satisfactory as allowed levels of total pollutants from an automobile exhaust into the atmosphere have been subsequently decreased by new regulations to levels almost approaching zero.
Exothermic energy released in the catalytic converter causes a temperature increase at the surface of the catalyst's substrate and also in the exhaust gases flowing past this surface. Monitoring of this temperature increase, caused by chemical exothermic energy release, provides another method for measuring the catalytic converter's overall chemical conversion performance. Two primary methods have been proposed for monitoring a catalyst's gas conversion capability by using a catalyst's temperature characteristics to determine its level of exothermic energy release. The first method uses two or more temperature sensors to passively monitor the temperatures of both the gases entering and exiting the desired portion of the catalytic converters being monitored. Within the catalytic converter, each catalyst element's outlet gas temperature increases to a steady state level above its inlet's gas temperatures depending on amounts of exothermic energy released at the catalyst's surfaces. Under some relatively steady state vehicle operating conditions, the temperature differences between the gases entering and exiting the portion of the catalytic converter being monitored provides an indication of the catalyst's condition. Methods covered by U.S. Pat. Nos. 5,592,815 and 5,630,315 apply this first method of catalyst temperature detection during periods of steady state engine operating conditions. The steady state temperature difference between the inlet and outlet catalyst temperature sensors, in a properly functioning catalytic converter, can be about 50-80° C. This may compare with a catalyst having insufficient conversion efficiency producing an indicated temperature sensor difference (outlet minus inlet) of between 10 and 40° C. Closely matched sensors, with stable, long-term error characteristics, are required to discern between good and bad catalytic converter conversion efficiency using such a technique.
A second method of temperature based catalyst monitoring uses a momentary disabling of the ignition system voltage to the engine's spark plugs to cause an unburned fuel and air mixture to exit the engine's exhaust. The time period of disabling the ignition system must be short in order to prevent the torque change from the engine to be noticed by the vehicle's driver. Engine operation typically must be selected during only lightly loaded conditions, such as during vehicle deceleration or engine over run, to prevent noticeable changes in engine smoothness to the vehicle's operator when disabling of the ignition occurs to generate an exhaust gas pulse with high levels of chemical energy. This pulse of unburned fuel and air mixture subsequently enters the catalyst and causes a sudden, small temperature rise in some portion of the catalytic converter's catalyst elements for a short time period. Quick responding temperature sensors are required to monitor this sudden and brief temperature rise at various portions within the catalyst where the unburned fuel and air mixture is oxidized exothermically at the catalysts' surfaces. Temperature sensors must also be placed at the proper location where the unburned exhaust pulse will be oxidized since most catalytic converters have multiple catalyst elements with a different precious metal catalyst makeup. The location where the pulse will be oxidized is dependent on many factors, such as the instantaneous temperatures of each catalyst element and the gas concentration levels at the catalyst's surface. Since engine operation may not be stable during the required light load engine enabling conditions, such as engine over run, the consistency of the chemical energy levels entering the catalyst may also be more variable than desired and degrade catalyst diagnostics accuracy. Identification of the instantaneous catalyst element temperatures and the location where the unburned exhaust pulse is oxidized can require multiple temperature sensors to be placed at various locations within the catalytic converter. Prior methods related to this second method are documented under U.S. Pat. Nos. 5,339,628, 5,435,172, 5,355,671, 5,610,844 and related documents.
A variation in this second method is disclosed in U.S. Pat. No. 5,715,676. In this disclosure the catalyst is mainly diagnosed with a catalyst temperature sensor during its warm-up phase shortly after cold starting by using an exothermic heating method employing rich A/F and a catalyst oxygen source from either a secondary air injection pump system or by operating some cylinders lean. However, the critical factor of when to enable the disclosed exothermic heating method, since a catalyst's characteristics are significantly changing in real time during warm-up as well as over the catalyst's operational lifetime, is not disclosed in U.S. Pat. No. 5,715,676. The diagnostic method proposed in this disclosure depend upon when in the catalyst warm-up phase, shortly after cold starting, that the diagnostic method disclosed is enabled and this is noted by its requirement that steady-state operating conditions should prevail for some time before and during the evaluation. However, some low thermal mass close-coupled catalyst systems can be fully heated in less than 30 seconds and this leaves little margin for error in the enabling time for the diagnostic method disclosed. An alternate solution proposed to resolve this critical enabling time issue in prior methods is by the use of a second catalyst temperature sensor to measure gas temperatures at the catalyst's inlet but adds more cost.
These two methods are dependent upon the ability of temperature detection devices or temperature sensors to be able to accurately detect small temperature differences with magnitudes between 10 and 40° C. or require very quick response characteristics. Monitoring of the catalyst's condition is typically performed during short time periods between 2 to 30 seconds when engine speed and load conditions are relatively stable. Normal temperature fluctuations, caused by engine exhaust gases entering the catalyst during the catalyst efficiency monitoring time period, are sometimes difficult to be accurately differentiated from temperature changes caused by the catalyst's exothermic reactions. Multiple temperature sensors are sometimes required with these methods to more accurately discern only temperature changes associated with the desired exothermic chemical activity. Other techniques, such as disclosed in U.S. Pat. No. 5,896,743, propose the continuous passive monitoring of high catalyst temperatures that can cause early deterioration of a catalyst's conversion efficiency performance. Accuracy of this technique is improved by incorporating additional methods to discern catalyst performance. One disclosed additional technique is determining the light-off temperature of a catalyst following a cold start by measuring the time to reach a predefined fixed temperature threshold that approximates initiation of exothermic catalytic activity. The method disclosed in U.S. Pat. No. 5,896,743 using only a single predefined fixed catalyst temperature threshold, for determining when a catalyst becomes active and begins generating exothermic heating, introduces a significant error in such diagnosis technique since the catalyst's activation temperature changes significantly over an automobile's useful lifetime. Many factors, such as contaminants of the catalyst's surfaces or condensation thereon and actual catalyst light-off temperatures, that vary over the automobile's lifetime, limit such catalyst diagnostic method's accuracy when using such fixed temperature thresholds and multiple unrelated testing methods.